This invention pertains to the art of metal surface treatment and, more particularly, to conversion coatings for plated metals.
The invention is particularly applicable to treatment or passivation of zinc-plated metal surfaces using a non-chrome conversion coating and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications such as treatment of other types of plated or non-plated substrates.
In the metal finishing industry, conversion coatings are used to provide metal surfaces with improved corrosion resistance. Conversion coatings also provide metal surfaces with improved adhesion for additional coatings such as paint or other finishes. A widely used conversion coating uses chromate. Such chromate conversion coatings are applied at various thicknesses, ranging from a very thin "blue-bright" finish to a very thick olive-drab finish. "Blue-bright" finishes are transparent with a slight blue tint and high luster. Such a finish not only imparts a corrosion-resistant coating to the surface of a substrate but also aesthetically enhances the substrate and articles made therefrom. Heavier chromate conversion coatings are considerably more protective than the bright finishes, but they do not meet the aesthetic criteria that are characteristic of the bright coatings. These heavier coatings are well-recognized by their yellow, bronze, or olive-drab finishes which are listed in general order of increasing film thickness.
Although conversion coating techniques using chrome provide satisfactory results, the techniques are undesirable because they necessarily include baths containing toxic hexavalent or trivalent chromium. Such chromium baths require special treatment before they can be disposed of. Therefore, the waste from the chromium based solutions raises grave environmental concerns that make their disposal prohibitive.
There are non-chrome treatments that form passivation coating but these are unsatisfactory for enhancing the corrosion resistance of a plated substrate, especially when the treated substrate is subjected to a moist environment. Such inferior treatments include phosphating treatments and treatments that have a bright dipping step followed by a coating step using a transparent lacquer. Each treatment provides only a barrier to corrosive conditions rather than an adhesive and cohesive film formed by chemical means between a film-forming element and a plated coating. Further, phosphating treatments provide barriers that are porous permitting moisture to pass through to the plated coating. Phosphating treatment coatings are not bright in appearance, but provide dull, opaque paint-like overcoats lacking the aesthetic attributes of chromium conversion coatings. Treatments that coat a bright-dipped article with a transparent lacquer typically use water emulsifiable polymer coatings that break down when wetted. Consequently, moisture penetrates to the bright dipped film, discoloring and diminishing its initial bright appearance.
Therefore, it has been deemed desirable to provide a chromium-free conversion coating that has the attributes of a traditional chromium conversion coating. Such a chromium-free system should be, as does a blue bright chromate conversion coating, impervious to and passive to moisture and pass a standard salt spray test for up to about 12 to 24 hours or more, while simultaneously enhancing the appearance of the substrate by imparting an attractive, brightly polished finish thereto. Such surfaces also should be more receptive to paint than the untreated plated surface.
An object of the present invention is to provide a conversion coating composition that does not contain hexavalent or trivalent chromium in order to eliminate the need for specialized waste treatment.
Another object of the present invention is to provide a conversion coating composition that results in an aesthetically pleasing finish.
Another object of the present invention is to provide a coating that offers a desirable level of salt spray protection.
The present invention contemplates a new and improved conversion coating composition which overcomes the above referenced problems and others and provides a coating which is chrome free, aesthetically pleasing, and corrosion inhibiting.